It is well known that the efficiency of supported catalyst systems is often related to the surface area on the support. This is especially true for systems using precious metal catalysts or other expensive catalysts. The greater the surface area, the more catalytic material is exposed to the reactants and the less time and catalytic material is needed to maintain a high rate of productivity.
Alumina (Al2O3) is a well-known support for many catalyst systems. It is also well known that alumina has a number of crystalline phases such as alpha-alumina (often noted as α-alumina or α-Al2O3), gamma-alumina (often noted as γ-alumina or γ-Al2O3) as well as a myriad of alumina polymorphs. One of the properties of gamma-alumina is that it has a very high surface area. This is commonly believed to be because the aluminum and oxygen molecules are in a crystalline structure or form that is not very densely packed. Gamma-Al2O3 is a particularly important inorganic oxide refractory of widespread technological importance in the field of important inorganic oxide refractory of widespread technological importance in the field of catalysis, often serving as a catalyst support. Gamma-Al2O3 is an exceptionally good choice for catalytic applications because of a defect spinel crystal lattice that imparts to it a structure that is both open and capable of high surface area. Moreover, the defect spinel structure has vacant cation sites giving the gamma-alumina some unique properties. Gamma-alumina constitutes a part of the series known as the activated, transition aluminas, so-called because it is one of a series of aluminas that can undergo transition to different polymorphs. Santos et al. (Materials Research, 2000; vol. 3 (4), pp. 104-114) disclosed the different standard transition aluminas using Electron Microscopy studies, whereas Zhou et al. (Acta Cryst., 1991, vol. B47, pp. 617-630) and Cai et al. (Phys. Rev. Lett., 2002, vol. 89, pp. 235501) described the mechanism of the transformation of gamma-alumina to theta-alumina.
The oxides of aluminum and the corresponding hydrates, can be classified according to the arrangement of the crystal lattice; γ-Al2O3 being part of the γ series by virtue of a cubic close packed (ccp) arrangement of oxygen groups. Some transitions within a series are known; for example, low-temperature dehydration of an alumina trihydrate (gibbsite, γ-Al(OH)3) at 100° C. provides an alumina monohydrate (boehmite, γ-AlO(OH)). Continued dehydration at temperatures below 450° C. in the γ series leads to the transformation from boehmite to the completely dehydrated γ-Al2O3. Further heating may result in a slow and continuous loss of surface area and a slow conversion to other polymorphs of alumina having much lower surface areas. Higher temperature treatment ultimately provides α-Al2O3, a denser, harder oxide of aluminum often used in abrasives and refractories. Unfortunately, when gamma-alumina is heated to high temperatures, the structure of the atoms collapses such that the surface area decreases substantially. The most dense crystalline form of alumina is alpha-alumina. Thus, alpha-alumina has the lowest surface area, but is the most stable at high temperatures. The structure of alpha-alumina is less well suited to certain catalytic applications, such as in the Fischer-Tropsch process because of a closed crystal lattice, which imparts a relatively low surface area to the catalyst particles.
Alumina is ubiquitous as supports and/or catalysts for many heterogeneous catalytic processes. Some of these catalytic processes occur under conditions of high temperature, high pressure and/or high water vapor pressure. The prolonged exposure to high temperature typically exceeding 1,000° C., combined with a significant amount of oxygen and sometimes steam can result in catalyst deactivation by support sintering. The sintering of alumina has been widely reported in the literature (see for example Thevenin et al, Applied Catalysis A: General, 2001, vol. 212, pp. 189-197) and the phase transformation due to an increase in operating temperature is usually accompanied by a sharp decrease in surface area. In order to prevent this deactivation phenomenom, various attempts have been made to stabilize the alumina support against thermal deactivation (see Beguin et al., Journal of Catalysts, 1991, vol. 127, pp. 595-604; Chen et al., Applied Catalysis A: General, 2001, vol. 205, pp. 159-172).
The research focusing on the thermal stabilization of alumina led to the development of high temperature-resistant materials such as hexaaluminates (Matsuda et al., 8th International Congress on Catalysis Proceedings, Berlin, 1984, vol. 4, pp. 879-889; Machida et al., Chemistry Letters, 1987, vol. 5, pp. 767-770) and the investigation of other potential oxide materials such as perovskites, spinels, and garnets, which have been examined with respect to both the thermal stability and catalytic performance.
Hexaaluminate structures have been shown to be effective structures for combustion catalysts because they provide excellent thermal stability and a higher surface area than alpha-alumina. Of particular interest, Arai and coworkers in Japan have developed hexaaluminates and substituted hexaluminates as combustion catalysts (Arai & Machida, Catalysis Today, 1991, vol. 10, pp. 81-95), and showed that the most promising stabilizer for combustion catalysts was barium (Arai & Machida, Applied Catalysis A: General, 1996, vol. 138, pp. 161-176). The investigation of the hexaaluminate material for the use of combustion has been described for example in Machida et al. (Journal of Catalysis, 1990, vol. 123, pp. 477-485) and in Groppi et al. (Applied Catalysis A: general, 1993, vol. 104, pp. 101-108). Machida et al. (Journal of American Ceramic Society, 1988, vol. 71, pp. 1142-1147) discovered that the crystal growth of one type of hexaaluminates, beta-alumina, also known as magnetoplumbite, was quite slow and anisotropic, and they proposed that its anisotropic growth may be the reason why the hexaaluminate can retain a large surface area at elevated temperatures. Arai and Machida (Catalysis Today, 1991, vol. 10, pp. 81-95) also disclosed that the thermal resistance of hexaaluminates seems to be quite dependent on the preparation procedures, primarily due to the difference of formation mechanism of hexaaluminates in various procedures. Kato et al. (Journal of American Ceramic Society, 1987, vol. 71(7), pp. C157-C159) disclosed a co-precipitation method to prepare mixtures of lanthanum and aluminum precursors, which resulted in formation of lanthanum beta-alumina structures with high surface area.
Destabilization of the support is not the sole cause of catalyst deactivation at high temperature. Stabilizing the catalytically active species on a thermally stable support is also needed. When an active species is supported on an oxide support, solid state reactions between the active species and the oxide support can take place at high temperature, creating some instability. That is why Machida et al. (Journal of Catalysis, 1989, vol. 120, pp. 377-386) proposed the introduction of cations of active species through direct substitution in the lattice site of hexaaluminates in order to suppress the deterioration originating from the solid state reaction between the active species and the oxide support. These cation-substituted hexaaluminates showed excellent surface area retention and high catalytic activity (see the hexaaluminate examples with Sr, La, Mn combinations in Machida et al., Journal of Catalysis, 1990, vol. 123, pp. 477-485). Therefore the preparation procedure for high temperature catalysts is critical for thermal stability and acceptable surface area.
It has long been a desire in the catalyst support arts to have a form of alumina that has high surface area like gamma-alumina and stability at high temperature like alpha-alumina. Such a catalyst support would have many uses.
One such use is in the production of synthesis gas in a catalytic partial oxidation reactor. Synthesis gas is primarily a mixture of hydrogen and carbon monoxide and can be made from the partial burning of light hydrocarbons with oxygen. The hydrocarbons, such as methane or ethane are mixed with oxygen or oxygen containing gas and heated. When the mixture comes in contact with an active catalyst material at a temperature above an initiation temperature, the reactants quickly react generating synthesis gas and a lot of heat. This very fast reaction requires only milliseconds of contact of the reactant gases with the catalyst. The combination of high exothermicity and very fast reaction time causes reactor temperatures to exceed 800° C., often going above 1,000° C. and even sometimes going above 1,200° C. Since catalysts used in the partial oxidation of hydrocarbons are typically supported, the support should be able to sustain this high thermal condition during long-term operation. In other words, a stable catalyst support which retains most of its surface area while enduring very high temperature, is desirable for long catalyst life.
The reaction pathway for partial oxidation of methane to synthesis gas is still being debated. Two alternate pathways have been proposed (Dissanayake et al., J. Catal., 1991, vol. 132, pp. 117; Jin et al., Appl. Catal., 2000, vol. 201, pp. 71; Heitne et al., Catal. Today, 1995, vol. 24, pp. 211).
These two pathways have come to be known as the combustion-reforming mechanism (Scheme 1), and the direct partial oxidation mechanism (Scheme 2). In Scheme 1, methane is completely oxidized to CO2 and water, and CO is a result of the reforming of water and CO2 with the residual methane. In Scheme 2, methane is pyrolyzed over the catalyst to produce CO directly without the pre-formation of CO2.
Weng, et al. (The Chemical Record, 2002, vol. 2, pp. 102-113) reported in situ Fourier transform infrared (FTIR) studies of the catalytic partial oxidation (CPOX) mechanism of methane over rhodium and ruthenium based catalysts supported on silica and alumina. They specifically studied the influence of the catalyst pretreatment conditions, and their relationship with the concentration of oxygen species on the surface of the catalysts under reaction conditions. They concluded that a) the CPOX mechanism, whether based on Scheme 2 (i.e., -direct oxidation) or based on Scheme 1 (combustion/reforming), is determined by the amount of O2- on the catalyst surface; b) an oxidized catalyst, such as Rh2O3, promotes the combustion/reforming mechanism (Scheme 1), whereas rhodium in the reduced state will promote the direct pathway (Scheme 2); c) rhodium on gamma-alumina under normal feed conditions of methane to molecular oxygen ratio in the feed will contain mostly oxidized Rh, even if rhodium was pre-reduced; d) the reducibility of rhodium is greatly affected by the support; and e) a lower reduction peak temperature, as measured by temperature-programmed reduction (TPR), indicates a weaker Rh—O bond.
A weaker Rh—O bond would lead to easier removal of the surface oxygen, and therefore the lower TPR temperature peak. During normal operating conditions, a weaker Rh—O bond should promote reduced rhodium on the surface, which would favor a direct pathway. In turn, this would lead to lower catalyst surface temperatures, which should slow the alumina phase transformation to ultimately alpha-Al2O3 (also slowing deactivation).
Roh et al. (Chemistry Letters, 2001, vol. 7, pp. 666-667) reported that nickel based partial oxidation catalyst based on theta-alumina had high activity as well as high stability, and they ascribed the excellent performance of these catalyst to the combination of the strong interactions between nickel and theta-alumina and the coexistence of reduced and oxidized nickel species. Liu et al. (Korean J. Chem. Eng., 2002, vol. 19, pp. 742-748) have also shown that a protective layer between Ce—ZrO2 and theta-alumina is formed to suppress the formation of nickel-aluminate spinel structures, which would result in catalyst deactivation. Moreover Miab et al. (Appl. Catal. A, 1997, vol. 154, pp. 17-27) indicated that the modification with an alkali metal (Li, Na, K) oxide and a rare earth metal (La, Ce, Y, Sm) oxide improved the ability of a nickel catalyst on alumina to suppress carbon deposition over the catalyst during partial oxidation of methane. Therefore the type of support used and the catalytic metal-support interactions are major factors in the catalyst stability and can have an effect on the reaction mechanism.
In addition to the selection and careful preparation of the support, catalyst composition also plays an important role in catalyst activity in catalytic partial oxidation of light hydrocarbons and selectivity towards to the desired products. Noble metals typically serve as the best catalysts for the partial oxidation of methane. Noble metals are however scarce and expensive, making their use economically challenging especially when the stability of the catalyst is questionable. One of the better known noble metal catalysts for catalytic partial oxidation comprises rhodium. Rhodium-based syngas catalysts deactivate very fast due to sintering of both catalyst support and/or metal particles. Prevention of any of these undesirable phenomena is well-sought after in the art of catalytic partial oxidation process, particularly for successful and economical operation at commercial scale.
It would therefore be highly desirable to create a thermally-stable high surface area support with a metal from Groups 8, 9, or 10 of the Periodic Table of the Elements (based on the new IUPAC notation, which is used throughout the present specification), particularly with rhodium, loaded onto said support for highly productive long lifetime catalysts for the syngas production, specifically via partial oxidation.